blush, v. — Green’s Dictionary of Slang

blush v.

blush like a black dog (v.) (also blush like a blue dog) [16C– mid-17C = black; subseq. is blue; a black/blue dog will not turn pink]

not to blush at all.

1579		S. Gosson School of Abuse (1868) 66: If it bee my fortune too meete with the learned woorkes of this London Sabinus, that can not playe the Poet without a Prompter; nor vtter a wise worde without a Piper; you shall see we will make him to blush like a blacke Dogge, when he is graueled.
1634		J. Withals Dictionarie in Eng. and Latine 557: Faciem perfricuit He blusheth like a black dogge, hee hath a brazen face.
1738		Swift Polite Conversation 7: col.: Fye, my Lord, you’ll make Mrs. Betty blush. lady sm.: Blush! Ay, blush like a blue Dog.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: To Blush like a blue Dog. Saying of any one incapable of Blushing.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: To blush like a blue dog, i.e. not at all.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1828		C.K. Sharpe letter Correspondence (1888) II 421: Dear Madam [...] I send you a pair of blue stockings of my own knitting. I blush like a blue dog about the workmanshjip, for I fear they are too short.
1919		Aberdeen Jrnl 29 Sept. 2/1: ‘The City Nightcap’ might have made the chaste muse of Aphra Behn [...] blush like a black dog.

I should blush to murmur

(US) a phr. of affirmation.

1885

‘High School Sl.’ in N.Y. Dispatch 31 May 7: ‘Did you catch on to the dude I mashed?’ ‘Well I should blush to murmer [sic]. He’s awfully jolly.’.